The present invention relates to CMOS image sensors, and more particularly, to CMOS image sensors that utilize capacitive transimpedance amplifiers.
In principle, CMOS image sensors provide a number of advantages over CCD image sensors. The CMOS yields are significantly better than those of the CCD fabrication process. In addition, the minimum noise levels achievable with CMOS-based sensors are substantially lower than those that can be obtained with CCDs.
Unfortunately, CMOS image sensors based on arrays of photodiodes suffer from a phenomenon known as xe2x80x9cbloomingxe2x80x9d. When light strikes a photodiode, electron-hole pairs are generated. The electrons are swept into the diode by the electric field across the diode. Normally the diode is reverse biased and the collected electrons reduce the potential across the diode by discharging a capacitor that is connected in parallel with the diode. Image sensor arrays measure the number of photoelectrons that are collected by each photodiode during a predetermined exposure period.
Consider an array of photodiodes in which one of the photodiodes is exposed to a strong light source. The photocurrent from that photodiode can become sufficiently large to discharge the capacitor and cause the photodiode to become forward biased. In this case, a large current flows through that photodiode from the amplifier to ground. Electrons from this current can reach neighboring photodiodes and be collected by the neighboring photodiodes. These electrons are indistinguishable from the photoelectrons that would be generated in the neighboring photodiodes if those photodiodes had been subjected to light. Hence, the neighboring photodiode appears to be irradiated with light also. Accordingly, a small, high-intensity, light irradiation pattern appears to xe2x80x9cbloomxe2x80x9d into a much larger pattern.
Broadly, it is the object of the present invention to provide an improved CMOS image sensor, which is less sensitive to blooming.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is an improved image sensor constructed from a photodiode and a transimpedance amplifier having an input connected to the photodiode. The present invention utilizes a clamping circuit to prevent the potential at the input of the transimpedance amplifier from causing the photodiode to become forward biased. In the preferred embodiment of the present invention, the transimpedance amplifier includes a transistor connecting the input to the output, and the clamping circuit includes a circuit for holding the base of the transistor at a clamping potential.